当指向前一个节点的指针不可用时,从单个链表中删除中间节点

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

def delete_middle_node(head: ListNode) -> ListNode:
    if not head or not head.next:
        return head

    slow, fast = head, head.next
    while fast and fast.next:
        slow = slow.next
        fast = fast.next.next

    slow.val = slow.next.val
    slow.next = slow.next.next

    return head